As Dude requested, the specific model number would be very helpful in offering additional advice because the 750w rating really doesn't tell the full story based on your needs. Specifically, I would be looking for a power supply that has all the correct connectors for my intended use. Now, in your case, it appears you are intending to crossfire two R9 270x cards. That will require multiple connectors that may not be available on all PSUs and this might force you to use one (or more) Y cables to complete the hardware installation. This chart says you would need two 6-pin connectors for each card. That means you would need 4 in total. See the specifications tab for the details.
Now, since Dude made the initial request for your PSU make/model, I assume he will do any further evaluation that is required (if you provide that information); however, since you are spending a nice piece of change on those two R9's, I wouldn't skimp on the power supply either.
Hope this additional information is helpful.
SpringUpdate
P.S. I also noticed you opened another thread looking for a "cheap" crossfire motherboard and to that I say "be careful about where you skimp" when building a new rig. Make sure your motherboard of choice has "all the features you will need" and then pay the price for it. For example, you may see two mobos that look very similar but one is cheaper. Why? Maybe it has less SATA3 ports or less PCI slots (if you would need them) or even USB3 ports. You need to clearly identify your own requirements first and then start shopping.